1. Field of the Invention
The present invention pertains to a method for inspection of ceramic monoliths for use in exhaust systems.
2. Description of the Related Art
Ceramic monoliths are widely used in exhaust system components as catalytic elements or as particulate traps. In common applications, one or a plurality of monoliths, generally in honeycomb form, are impregnated and/or coated with catalytic materials, then packaged into a metal “can”.
The manufacture of such monoliths include numerous steps such as forming by casting or extrusion methods, drying, sintering, impregnating, wash-coating, firing, etc. Damage to the monolith may occur in each of these steps. While some damage is external and immediately visible, internal damage is not, and if gone unnoticed, may result in production of a defective exhaust catalyst element, or one which may later fail. In monoliths with internal damage, the hot/cold cycles which the monoliths will experience result in deterioration of the monolith. Internal damage in components such as particulate filters can allow particle laden exhaust gas to flow through the filter without any filtering. Thus, it would be desirable to be able to determine whether ceramic monoliths for exhaust systems have any internal damage. It would further be desirable to be able to detect such damage at multiple stages during the manufacturing process, for example both prior to and after catalyst deposition.
Ultrasonic imaging has been used in numerous fields to determine product shape, and in some cases, internal damage. An example is the measurement of impact damage on quasiiotropic laminates for aerospace applications (so-called “c-scan”). A further example is disclosed in U.S. Pat. No. 6,439,054, where ultrasonic imaging is employed to test homogeneity of sputtering target materials. Both these methods, however, require application of ultrasonic energy in a water bath, which is not satisfactory for use with porous and water absorbent ceramic materials.
Corning U.S. Pat. No. 4,557,773 (“Corning”) discloses the use of ultrasound to determine the location of alternating open and closed passageways in a monolithic diesel particulate filter (“DPF”) after a ceramic end cap has been applied by directing ultrasound into the monolith and detecting transmitted ultrasound at the opposite face, while Corning U.S. Pat. No. 4,752,516 discloses the use of ultrasound to assist in the introduction of polymer into cells to form a mask. However, neither of these patents discusses detection of flaws. U.S. published application 2005/0247131 and U.S. Pat. No. 5,964,694 disclose the use of ultrasound to determine the degree of plugging of DPFs. However, no scan is performed, only a test based by changes in reflected ultrasonic energy.
In U.S. published application 2006/0137525, Corning, Inc. a pioneer in monolith production and testing, discloses several methods of testing plugged honeycomb structures such as are commonly used for diesel particulate filters. However, despite the availability of ultrasonic methods for use in other areas of technology, use of ultrasonic testing is not disclosed. Rather, tests such as monitoring the pass through of graphite powder, the through-flow of soot-containing gases, and use of temperature sensitive LCP films while blowing cool air through the monolith, have all been used. In the '525 application, ultrasonic energy is used in an ultrasonic humidifier to produce a vapor which flows through the honeycomb and is then later is detected downstream from the honeycomb.
One reason that ultrasound has not been used for testing ceramic monoliths for the presence of flaws may have to do with their structure, which is generally a multiplicity of parallel passages (cells) with porous ceramic walls. Aiming ultrasound through these cells and detecting the transmitted sound at the opposite face of the monolith may detect plugged or collapsed cells, but may not always detect flaws such as holes in the cell walls, cracks, etc. Aiming ultrasound into the cell and observing the reflected signal would be expected to be associated with such a variety and number of internal reflections/re-reflections that such a technique would not be considered viable.